I’m thinking of trying to make some custom cabinet knobs for my
kitchen. I’d like to carve them in wax and cast them in bronze,
either Everdur silicon bronze or Rio’s Ancient Bronze alloy. Rather
than messing with trying to tap out screw threads for a couple dozen
knobs, I was thinking of embedding stainless steel threaded inserts
into the wax and casting the bronze around them. I’ve never done
anything like this before–will it work?

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I was thinking that if I embedded the steel threaded insert in the
wax model, invested it and ran it through a burn-out cycle in the
kiln, the steel would be plenty hot when I cast the bronze. Is this
not correct?

I was thinking that if I embedded the steel threaded insert in the
wax model, invested it and ran it through a burn-out cycle in the
kiln, the steel would be plenty hot when I cast the bronze. Is
this not correct?

Yes, the steel will be hot. But don’t expect this to mean the bronze
will fuse or braze itself to the steel. The steel may get hot, but it
also will oxidize, so while the bronze will cast to the shape of the
steel intimately enough, it probably won’t strongly bond to it. As
well, the different thermal shrinkage rates of the steel versus the
cast bronze might affect a bond too. What you need to do is to first,
when you set the steel into the wax, be sure that the exposed parts
of the steel are able to be held in place by the investment, so the
steel won’t shift once the wax is burned out. Then, you also need to
have made sure that the shape of the steel insert is such that the
bronze will “key” to it, meaning the steel is mechanically prevented
from moving or loosening by the shape of the bronze around it, rather
than any bond. For example, if the steel insert is a straight round
bar sticking into the bronze (or for that matter, a threaded one
too), if you cast it like that, but steel can twist or pull itself
out if forced. But if you had drilled a hole or two crosswise through
the rod, or put some flat areas on the rod that the bronze would fill
into, then the rod is trapped, and cannot move, even if the bronze
has not bonded to it.

I was thinking that if I embedded the steel threaded insert in the
wax model, invested it and ran it through a burn-out cycle in the
kiln, the steel would be plenty hot when I cast the bronze. Is
this not correct?

Might be, but I’d invest the whole piece of steel to retain the
heat, if you didn’t the steel on the outside of the investment would
draw the heat out of the mould.

If you gravity pour, it will be interesting, if you use a
centrifugal it will be interesting, vacuum may be the best option.

Forgive me for saying this but this is a very poor idea. First, to
answer your question, yes the steel would be hot after burnout but
it would also be heavily ladened with firescale. This will make sure
that the threaded rod will not be firmly encased in the
bronze.Second, what’s going to happen down the road when you try to
fasten this thing down and the threaded rod just spins around; or,
at some point breaks and needs to be replaced? I guess I don’t really
understand your aversion to drilling and tapping your castings. It’s
really not hard to do; I think someone already posted a link to a
tutorial. Drilling and tapping your castings will insure a happy
result; the other way is only going to lead to problems.

Drilling and tapping is the best bet, I’d say. Should you wish to
persist…

Main problem with your plan is to keep the steel component ready for
fusion over the period it takes to heat it to that temperature. I
have found also that the flux that would normally protect the steel
from oxidation is ‘used up’ over that period, with no hope of
topping up due to the closed nature of the investment void. I have
noticed that the investment dissolves in the flux at casting
temperature thus loosing the fidelity of the shape being cast. You
could try tinning the embedded end of the stud or screw with layer of
bronze or silver solder. This would give you a fusion bond with the
steel and (erm…hopefully) the incoming casting metal could have a
greater change of bonding with the tinned surface than the original
steel. Now I want to go try this out myself!

However, I have fixed steel screws into aluminum castings for the
same purpose and have found that if you arrange to cast round the
head of a machine screw, preferably csk slotted, the shape of the
head is enough to form a mechanical key with the surrounding
aluminum matrix. There is little or no chance of a fusion bond
between aluminum and steel with my set up. This will at least cut out
the need for crosswise drilling of the stud.

Similarly, I have attached aluminum castings to a larger mixed metal
component using steel nuts welded/brazed edgewize to form loops
through which the incoming casting alloy flows.

I have spent many years casting copper alloys around steel
component, and have just uploaded some pictures of work with embedded
screws. I use these as purely decorative shapes. No business end
poking out though!

The way I have affixed screws to the back of bigger metal objects to
be mounted is to use a flat topped csk screw. There is enough area on
the top surface of the screw to allow for a silver solder bond
between screw and object. Maybe this would not be strong enough for a
high load job like a cabinet handle.